Molding epoxy resin materials are conventionally in wide use for the encapsulation of device elements in electronic component parts such as transistors and ICs (integrated circuits). This is because the epoxy resin has properties such as molding properties, electrical properties, moisture resistance, heat resistance, mechanical properties and adhesion to component inserts which are all well balanced. In particular, a combination of o-cresol novolak epoxy resin with a phenolic novolak resin is superior in view of the balance of these properties, and is widely used as base resins of encapsulation molding materials.
In recent years, with progress of high-density packaging on printed circuit boards of electronic component parts, conventional through-hole mount packages such as DIP (dual in-line package) have become less used, and small-sized thin surface mount packages such as QFP (quad flat package), SOP (small out-line package), TQFP (thin quad flat package) and TSOP (thin small out-line package) have become more used. The surface mount packages are different in the manner of packaging from the conventional through-hole mount packages. Device elements are provisionally fastened to the wiring board surface and thereafter soldered using a solder bath or a reflowing tool, and hence the whole package is inevitably exposed to soldering heat. Thus, in the case of the surface mount packages, when packages stand moisture-absorbed, the absorbed moisture vaporizes and expands abruptly upon heating at the time of soldering, bringing about the problem of separation at adhesion interface or package cracking.
Accordingly, in order to solve such a problem, the improvement of molding materials has been made energetically. In order to avoid the package cracking during the reflowing process, the epoxy resin, and curing agents of a resin component of molding materials have been reconsidered. As a result, various molding materials compounded with a biphenyl skeleton epoxy resin having a lower hygroscopicity and better adhesive properties than the conventional o-cresol novolak epoxy resin are proposed (e.g., Japanese Patent Applications Laid-open (KOKAI) No. 64-65116 and No. 3-207714). At present, molding materials compounded with this biphenyl skeleton epoxy resin have begun to be put into practical use as materials for the encapsulation of surface mount packages, and have greatly contributed to the improvement in reflow resistance of the surface mount packages.
The molding materials compounded with the biphenyl skeleton epoxy resin have a reflow resistance remarkably superior to that of molding materials compounded with the conventional o-cresol novolak epoxy resin, but have a problem that the resin has a low glass transition point (Tg). Thus, there is a limit in application. Accordingly, a resin having much lower moisture absorption and higher adhesion has been sought.